Volume 4, Issue 1

A few microns thick layer of an optically active cholesteric liquid crystal is realized by using a photoisomerizable nematic component and a chiral dopant. It is shown that such a photosensitive optically active medium can be used as a holographic material for optical information and dynamic grating recording. The photo-induced gratings are written by exploiting the light-induced photoisomerization phase transition from an optically active chiral liquid crystal to an isotropic liquid, which results in the rotation of the light polarization plane from 90 to 0 degrees and corresponding to maximum, respectively, zero transmittance. The results highlight applications in the field of optical storage by the recording of static gratings, as well as in the feld of nonlinear beam-coupling via the holographic writing of dynamic gratings

Photovoltaic fields induced in x-cut Fe-doped lithium niobate (Fe:LiNbO 3 )were used to achieve optically induced defect formation and light valving in a vertically aligned nematic liquid crystal. Initially, the optical axis of the LC was vertically aligned (along the surface-normal of the planar, photovoltaic substrates) throughout the whole sample. Samples were exposed with a focused continuous wave laser beam and investigated via microscopic imaging in-between crossed polarizers. The optical axis of the planar, x-cut Fe:LiNbO 3 substrates was in the substrate plane and oriented parallel to one of the polarizers, which resulted in an initially dark state. Optically induced surface fields (with high in-plane components) generated within the substrates led to director reorientations and defect formation. Accordingly, the samples were locally switched into a transmissive state. The area affected by exposure was larger (300 μm) than the FWHM of the Gaussian exposure beam (14 μm). Switching from dark to bright states (light valving) could be achieved in the investigated samples much more eficiently than in previously investigated samples with z-cut Fe:LiNbO 3 -substrates. Realignments of the LC director were induced at lower optical power density (140 mW/cm 2 ) than would be required to excite the intrinsically present nonlinear optical responses in a nematic LC such as the light induced Fredericks transition.

Liquid crystal cells based on lithium niobate substrates have recently been proposed as good candidates for optofluidic devices and for light-induced controlled generation of defects in liquid crystal films. The peculiarity of these structures lies in the possibility of using the bulk photovoltaic effect of lithium niobate to obtain an optically induced dc field able to affect the molecular liquid crystal director. Reversible fragmentation and self-assembling of liquid crystal droplets driven by the lithium niobate pyroelectric properties have also been reported. We review the basic results obtained so far with the aim of making the point and seeing what else can be done in the framework of the realization of hybrid structures combining lithium niobate with the electro-optical and nonlinear optical properties of liquid crystals.

Smart surfaces with preferably reconfigurable wetting properties can lead to key applications in labon- a-chip analytical and preparative systems. In this paper, we present our recent results obtained using polymer surfaces whose wetting properties are modified in a permanent manner using laser ablation and in a reconfigurable manner using plasma oxidation. Polydimethylsiloxane (PDMS) diluted in solvent is used as the polymeric material coated over microscope glass slides in our studies. In the first part, the tracks of ~ 70 μm width are defined over the surface by surface oxidation using cold plasma exposure through a microfluidic channel. In the second part, femtosecond laser micromachining is used for selective removal of polymer coating and uncovering the hydrophilic glass substrate. We experimentally demonstrate guiding of water in the form of filaments and droplets over the obtained hydrophilic tracks of ~ 110 μm. We also discuss preliminary experiments to coat light sensitive azobenzene over a glass substrate with the help of a silane in order to achieve reversible isomerization upon periodic exposure to UV/vis radiation. Furthermore, we elaborate advantages, challenges and the significant role of such patterned surfaces in future applications.

Photonic sensors receive an increasing global attention focusing on critical applications such as the protection of the environment, the health and safety of the citizens. In this work we are concerned with sensing of gaseous environments and focus on a novel class of nanocomposite sensor materials, which are capable to react to external stimulant-agents. These interactions amend their optical properties, fact which provides the means for optical detection. An unpowered sensor device is thus formed, which is interrogated optically and remotely to provide quantitative detection of the “agent”. “Sensing-by-light” thus unfolds the emerging conceptsof “Remote-point Photonic Sensors (RPHOS)”,we review and revisit in this work. The method offers unique means for spatially localized, real-time, multivariable sensing. This “point-in-space” detection may be contrasted to opticalpath- integrating LIDAR operations. Synergy with diffractive and holographic approaches enhance detectability and offer further innovative modes of operation. The low cost of this unpowered sensor-head, the flexibility of fabrication, the capacity for parallel / multiplexed/ data transmission and the immunity to electromagnetic interference, makes this technology ideal for use in harsh and adverse environments of industrial and security applications.